A common problem of defect detection is distinguishing between defects and noise. Typically, detection of defects on semiconductor masks, reticles and wafers is done by comparing an image of a portion of an article (e.g., mask, reticle or wafer) with a reference image. In a Die-to-Model method, the presence or absence of a defect in a location is checked by comparing the pattern at the desired location at an inspected die with the pattern of the same location in a model, prepared based on, for example, die design data or a model of inspected die/s. In the Die-to-Die method, the presence or absence of a defect in a location is checked by comparing the pattern at the desired location at an inspected die with the pattern of the same location in an identical die, for example, a previously inspected die on the same mask or wafer (Inter-Die comparison). Since probability of having same defect in the same location on two different dies is practically close to zero, significant dissimilarities between two dies are considered defects.
Typically, in the Inter-Die comparison method, the data to be compared is collected at two spaced apart locations—each located at a different die. While the two locations—in the inspected die and reference die, correspond to the same pattern, each is subjected to different variations, for example, process variations, mechanical and electrical variations. Such variations impose noise, for example, in a Difference image (“Diff”), which is an image created by, for example, subtracting the inspected image from the reference image. This noise should be considered and disregarded; otherwise detection sensitivity and integrity are hindered. This challenge increases as the design rules shrinks.
There is a need for sensitive and accurate die-to-die defect detection techniques.